He certainly wasn’t alone. Admiral Nagumo, who commanded the fleet which attacked Pearl Harbor, predicted “The success of our surprise attack on Pearl Harbor will prove to be the Waterloo of the war to follow.” (Given that Pearl Harbor was the opening attack in nearly four years of war and that Waterloo was the decisive end to Napoleon’s wars, that comment is somewhat difficult to understand, although its import is clear enough. Maybe it lost something in translation.)
Yamamoto said in September 1941: “It is a mistake to regard Americans as luxury loving and weak. I can tell you that they are full of spirit, adventure, fight and justice. Remember that American industry is much more developed than ours and - unlike us - they have all oil they want. Japan cannot vanquish the United states. Therefore we should not fight the United States.”
Source for above quotes is Pacific Fury, Peter Thompson, Random House Australia, 2009, pp. 8-9
He was astute to the point of accurate prophecy.
The Japanese, it must be emphasized, did not seek the total defeat of the United States and had no intention of invading this country. They planned to fight a war of limited objectives and having once secured these objectives to set up a defense in such depth that the United States would find a settlement favorable to Japan an attractive alternative to a long and costly war. To the Japanese leaders this seemed an entirely reasonable view. But there were fallacies in this concept which Admiral Yamamoto had pointed out when he wrote that it would not be enough “to take Guam and the Philippines, not even Hawaii and San Francisco.” To gain victory, he warned his countrymen, they would have “to march into Washington and sign the treaty in the White House.” [49] Here was a lesson about limited wars that went unheeded then and is still often neglected.
(My emphasis, to underline the distressing failure of various belligerents around the world, and the USA in particular, to heed Yamamoto’s wisdom and the comment of the US WWII official history authors close to sixty years ago, preceding predictable failures in limited wars in Vietnam, Iraq and Afghanistan, among others.)
http://www.history.army.mil/books/70-7_04.htm
Having made the grievous mistake of disposing of almost all of my many books some years ago, I can’t lay my hands on a published source for the “run wild for six months” quote.
From imperfect memory, I think Yamamoto made recorded comments to the same effect on several occasions, including at senior government level when proposals for war were being considered, and or in response to the decision to go to war, and similar comments to other senior naval officers and other senior people in government.